This invention relates to image data compression for telecommunication of a facsimile of a two-dimensional image and more particularly, to an apparatus for selecting a reference line for vertical mode coding of image data.
Equipment for telecommunication of images include a transmitter at one location for encoding an image, a communication link for communicating the encoded image data, and a receiver at another location for decoding the encoded image data. Typically, the major cost of facsimile transmission is the cost of transmitting the image data over the communication link, such as a telephone line. Preferably, the image data is compressed prior to transmission to reduce the number of bits of image data to be transmitted and thereby reduce the operating cost of the communication link.
A known method for compressing image data is to encode the image data using a vertical mode coding technique described in "Standardization Of Group 3 Facsimile Apparatus For Document Transmission". CCITT Recommendation T.4 (Geneva, 198O) which is incorporated herein by reference. Vertical mode coding techniques require a reference scan line for each scan line of the image to be encoded for transmission. The reference scan line is used so that the current scan line to be coded can be represented by coding only the changes or differences between the current scan line and the reference scan line.
Invariably, the CCITT facsimile data compression coding algorithms use the scan line immediately preceding the current scan line as the reference scan line. However, since the differences between the current scan line to be coded and the immediately preceding scan line can be great, the amount of image data transmitted over the communication link can be substantial. This often frustrates the purpose of image data compression.